Dielectric insulation media in liquid or gaseous state are conventionally used for the insulation of an electrically active part in a wide variety of electrical apparatuses, such as switchgears or transformers.
In medium or high voltage metal-encapsulated switchgears, for example, the electrically active part is arranged in a gas-tight housing, which defines an insulating space, said insulation space comprising an insulation gas usually at several bar pressure and separating the housing from the electrically active part thus preventing flow of electrical current between housing and active parts. Metal-encapsulated switchgears allow for a much more space-saving construction than switchgears which are mounted outdoors and are insulated by ambient air. For interrupting the current in a high voltage switchgear, the insulating gas further functions as an arc extinction gas.
Many of the conventionally used insulation media have several drawbacks.
On one hand, conventional insulation gases with high insulation and switching performance often have some environmental impact when released into the atmosphere. So far, the high global warming potential (GWP) of these insulation gases has been coped with by strict gas leakage control in gas-insulated apparatuses and by very careful gas handling.
On the other hand, conventional environment-friendly insulation gases, such as dry air or CO2, have quite a low insulation performance, thus requiring the gas pressure and/or the insulation distances to be increased.
For the reasons mentioned above, efforts have been made in the past to replace the conventional insulation gases by suitable substitutes.
For example, WO 2008/073790 discloses a dielectric gaseous compound which—among other characteristics—has a low boiling point in the range between −20° C. to −273° C., is preferably non-ozone depleting and has a GWP of less than 22,200 on a 100 year time scale. Specifically, WO 2008/073790 discloses various different compounds which do not fall within a generic chemical definition.
Further, U.S. Pat. No. 4,175,048 relates to a gaseous insulator comprising a compound selected from the group of perfluorocyclohexene and hexafluoroazomethane, and EP-A-0670294 discloses the use of perfluoropropane as a dielectric gas.
EP-A-1933432 and co-pending US-A-2009109604 refer to trifluoroiodomethane (CF3I) and its use as an insulating gas in a gas-insulated switchgear.
The use of a compound of general formula CxHyFzI in general, and of CF3I in particular, as an insulating medium for electric power transmitting and distributing machines is further disclosed in EP-A-1146522.
However, despite of the insulation properties mentioned in these documents, CF3I has a relatively low thermal stability: at around 100° C. it starts to decompose into hazardous products, including I2, which can form a solid, conducting residue.
Considering the drawbacks of the insulation media that are state of the art, and in particular those of CF3I, the object of the present invention is thus to provide a thermally stable dielectric insulation medium which has a low impact on the environment.